doc rearrangement

[Graham Percival]

I forgot to mention that the main links to manuals are broken, but
if you follow the "split HTML" links, they work.


Anyway, in addition to the webpage, the documentation has been
slightly reorganized.  In particular
- the beginning of the Learning manual has been streamlined.
    (although this is another area that could use helpful users
     submitting screenshots or something)
- the last chapter of the Learning manual was moved to Usage
- Usage in general has been beefed up
- we have a dedicated Essay about computer music engraving

If anybody has a major problem, or suggestion, about the
documentation, I'd like to hear about it soon.  In a week or so,
barring any major problems, we'll declare the current setup `good'
and go from there.

Cheers,
- Graham